The present invention relates to a solid-state laser oscillator which can stably generate a laser beam with a high power output and exhibiting a high quality factor (Q), and a machining apparatus using the same.
FIG. 5 is a schematic view showing the configuration of an oscillator section of a conventional solid state laser device which has been used to oscillate a laser beam with high quality (Q). In FIG. 5, reference numeral 1 denotes a rod-like solid-state element (host), e.g., a yttrium aluminum garnet (YAG) crystal doped with neodymium (Nd), i.e., Nd: YAG laser. Reference numeral 2 denotes an excitation light source, e.g., Krypton arc lamp, Xenon flash lamp, etc. Reference numeral 4 denotes a condenser formed so as to enclose the solid-state element 1 and excitation light source 2. Numeral 31 denotes a partial reflecting mirror and numeral 32 denotes a total reflecting mirror.
FIG. 6 is a sectional view of a laser oscillator which is directed to the prior art for stabilizing a laser oscillator with high quality Q as disclosed in Solid-State Laser Engineering, 2nd Edition, Springer-Verlag, pp. 192 to 193. Reference numerals 11 and 12 denote first and second rod-like solid-state elements, respectively, and reference numerals 21 and 22 denote first and second excitation light sources, respectively. Reference numeral 31 denotes a partial reflecting mirror; 32 a total reflecting mirror; 60 a 90.degree. crystal optical rotator; and 61 a Brewster window.
Referring to FIG. 5, a conventional laser oscillator is described as follows. It is well known that the quality of a laser beam improves as the ratio of a beam in a solid-state element to that of a Gaussian beam calculated theoretically in the solid-state element decreases. To increase the laser output, the length of the resonator can be increased, or an aperture can be made in the resonator to restrict oscillation to the lowest order of transverse mode (TM.sub.00) in the solid-state laser cavity, and thereby obtain a high Q. An "offset" laser resonator can be employed to boost output power and energy by using a reflecting mirror with a small radius of curvature, e.g., lm or less, typically 0.1-0.5 m, where the laser beam is converged to a small spot at the front surface of the reflecting mirror. A convex partial reflecting mirror can also be arranged in the vicinity of the solid-state element to form a reflecting mirror, increasing the effective length of the laser cavity by substantially several meters, taken together with a convex thermal lens effect produced by the solid-state element.
In an experiment by the inventors of the present invention, where the resonator was structured as above, such that the diameter of the Gaussian beam theoretically calculated in the vicinity of the solid-state element was increased to, e.g., about 1/5 the diameter of the solid-state element, a laser beam with high quality (Q) could be obtained that was about 1/20 the refraction limit, i.e., a transverse mode order of about 20 that is about 1/10 that of a normal laser oscillator.
However, this resonator structure has the problem in that it gives rise to a reduction of oscillation efficiency and fluctuation in the laser output. This is notable in the case where the resonator is operated with a high output of 100 W or larger, in which distortion of the solid-state element is increased. This tendency to distortion is noticeable as the quality of the beam is increased.
FIG. 7 graphically shows one example of the oscillation characteristic acquired in an experiment on the conventional solid-state laser oscillator. In the graph, line A illustrates the oscillation characteristic of a laser beam with poor beam quality, e.g., about 1/200 times a theoretical limit, i.e., having the transverse mode order of 200, and line B illustrates that of a laser beam with high beam quality having the transverse mode order of 20. The line B does not exhibit a linear oscillation characteristic but a curved characteristic including several peaks. It can be seen that the fluctuation of the output is notable at the sections where there are valleys in the oscillation curve, i.e., under the condition where the output is relatively low.
On the other hand, in the prior art shown in FIG. 6, it is known that with a first and a second rod-like solid-state element 11 and 12 arranged in tandem, and a 90.degree. crystal rotator 60 located at the center between these solid-state elements, if the incidence of birefringence generated by the first solid-state element is canceled out by the second solid-state element, a laser beam with a stabilized output and with high efficiency can be obtained. Specifically, birefringence refers to the effect of causing two polarization components orthogonal to each other to discern different refractive indices owing to thermal stress generated in the solid-state element. Thus, the laser beam incident on the birefringent solid-state element will discern either one of two kinds of thermal lenses according to its polarization direction.
The 90.degree. crystal rotator 60 rotates the polarized light of the laser beam which has permeated through the first solid-state element 11, and causes it to be incident on the second solid-state element 12. Thus, the laser beam incident on the first solid-state element 11 equally discerns two kinds of thermal lenses when it has passed through both solid-state elements. Accordingly, with the laser beam polarized in either polarization direction, and with birefringence being exhibited by both the solid-state elements, the laser beam discerns both thermal lenses in the combination of the two solid-state elements, and two polarized beams oscillate under substantially the same conditions to provide an effect as if the separation of polarized light by birefringence of the solid-state elements has been canceled out.
The prior art shown in FIG. 6 was designed to drive a linear polarized light efficiently and stably. In addition, according to the experiment carried out by the inventors of the present invention, it was also confirmed that in a resonator in which linear polarized light is not the objective and a Brewster window is not present, careful insertion of a 90.degree. crystal rotator 60 between two solid-state elements 11 and 12 to cancel the influence of birefringence can improve the efficiency of the laser oscillation. The oscillation characteristic illustrated by curve C as shown in FIG. 7, provides an oscillation waveform with no substantial fluctuation.
A theoretical explanation follows regarding the difference between the prior art shown in FIGS. 5 and 6. First, in the prior art shown in FIG. 5, the solid-state element is excited by the environment and becomes thermally deformed. For example, the solid-state element with a circular section provides a difference in extension of the crystal and change in the refractive index between a diameter direction and a radial direction of the section. The directions of the two extensions are orthogonal to each other, to provide two lens functions in the respective directions. Therefore, the laser beams having two basic polarization modes; i.e., polarization components shown in FIGS. 8A and 8B, are subjected to different dimensions of lens function when they pass through the solid-state element. For this reason, the diameters of the Gaussian beam theoretically calculated within the solid-state element, for the output of an excitation light source, can be plotted as two curves of B1 and B2, as shown in FIG. 9B, for the laser beams having the basic polarization modes shown in FIGS. 8A and 8B. The section represented by the two curves with diameters B1 and B2 of the Gaussian beam, is an area where oscillation can occur, which can be calculated for the respective basic polarization modes. In the other areas, oscillation does not occur owing to great losses in the resonator.
A comparison between the oscillation characteristic represented by curve B in FIG. 7 and the oscillation area shown in FIG. 9B, shows experimentally that the oscillation characteristic shown by curve B in FIG. 7 has three outputs with peaks whose sections are coincident to oscillation areas B11, B12 and B22 in FIG. 9B. This means that a high output and resonator stability can be obtained only under the condition where two polarization mode beams oscillate in a completely overlapping manner, or where only one polarization mode oscillates stably whereas the other polarization mode is located in an unstable oscillation area. This is attributable to the following fact.
For example, under the boundary condition between B11 and B12 in FIG. 9B, transition from the basic polarized light mode to the other polarized light mode, occurs owing to fluctuation or non-uniformity of distortion of the thermal lens in the solid-state element. Thus, resonation in the polarization mode, with great laser cavity losses, also partially occurs so that gains by the solid-state elements can be canceled out by each other. As a result, the resonator losses are increased to lower the laser output and make the resonator unstable.
On the other hand, in the prior art shown in FIG. 6, the oscillator conducts the polarization conversion by means of a polarized rotator (90.degree. crystal rotator 60) inserted between the two solid-state elements 11 and 12, thereby canceling any difference between the resonating modes in the two kinds of polarization modes. The oscillation area results as shown in FIG. 9A. The oscillation areas A1 and A2 in both polarization modes substantially overlap each other. In this way, both polarization modes uniformly oscillate with low cavity losses so that a stabilized oscillation can be obtained with a high efficiency, as shown in curve C in FIG. 7.
As described above, in the prior art, in order to improve the quality of a laser beam, the oscillator was so configured that the ratio of the diameter of the beam in a solid-state element to that of a Gaussian beam theoretically calculated in a solid-state element, was small. But at a high laser output of 100 W or more, the oscillation efficiency would deteriorate and the output would fluctuate due to birefringence. Further, in order to obviate such a disadvantage, a technique for canceling out the effects of birefringence by inserting a crystal rotator between two solid-state elements arranged in tandem, was adopted. But, for this purpose, two solid-state elements having substantially the same quality were required, and a technique of accurately arranging the solid-state elements on an optical axis was also required.